1. Field of the Invention
The present invention relates to an optical fiber transmission line, more particularly relates to an optical transmission line comprised of optical fibers for wavelength division multiplex (WDM) transmission.
2. Description of the Related Art
With optical fiber transmission lines built using optical fibers being increased in speed and capacity, WDM transmission has come to be viewed as an important technology for commercial application. The new disadvantage as non-linearity, however, arises in the optical fibers due to the higher power of the optical signals and the interaction among signal wavelengths.
Among the non-linearity phenomena, four wave mixing (FWM) has particularly serious repercussions in WDM transmission. Intensive studies are underway on methods for suppressing this. FWM is remarkable between optical signals of wavelength regions with small dispersion, so small dispersion optical fibers are known to be disadvantageous in this regard. Even the conventionally often used Non-Zero Dispersion Shifted Fibers (NZ-DSFs) having absolute values of dispersion of 2 to 8 ps/nm/km or so have been insufficient.
Further, the waveform distortion caused by self-phase modulation (SPM) or cross-phase modulation (XPM) is also a very serious disadvantage. The distortion "PHgr"NL of a signal due to SPM or XPM is generally expressed by equation (1). In equation (1), n2 shows the non-linear refractive index, Aeff shows the effective core area, and xcex shows the transmission wavelength:
"PHgr"NL=(2xcexn2xc3x97Leffxc3x97P)/(Aeff)xe2x80x83xe2x80x83(1)
Therefore, along with studies to keep down the non-linear refractive index n2 as reported in OFC""97TuN1b etc., the expansion of the effective core area Aeff has long been studied as reported in OFC""96WK15 and OFC""97TuN2. The expansion of the Aeff, however, was found to easily cause the disadvantages of an increase in the bending transmission loss, an increase in the dispersion slope, etc. in an optical fiber of a type making up a transmission line on its own such as with the conventional NZ-DSF. An NZ-DSF type transmission line is therefore also problematic in this regard.
To overcome the above disadvantages, it has been proposed to manage the dispersion for an overall transmission line built using optical fibers.
For example, Japanese Unexamined Patent Publication (Kokai) No. 9-211511 discloses a connection of a low non-linearity optical fiber having a positive dispersion and a high non-linearity optical fiber having a negative dispersion along the direction of transmission of the optical signal in the order of the low non-linearity optical fiber and the high non-linearity optical fiber to obtain the optimal optical transmission line. As specific examples, as disclosed in ECOC""97, vol. 1, p. 127, a single mode optical fiber (SMF) having a positive dispersion characteristic and a transmission line dispersion compensation optical fiber (transmission line reverse dispersion optical fiber (RDF)), having a reverse dispersion characteristic, have been proposed. These SMF and RDF have large local dispersions of about 16 to 22 ps/nm/km in terms of absolute values, so are advantageous in terms of suppressing the FWM.
Even if the FWM can be suppressed by using the above SMF or RDF, however, a dispersion of over 16 ps/nm/km accumulate in the longitudinal direction, so wavelength distortion occurs due to the dispersion in the optical transmission line. This large cumulative dispersion becomes a particularly large disadvantage at the time of high speed transmission.
Further, with an RDF having such a large dispersion value, the transmission loss generally ends up becoming more than 0.24 dB/km. Further, the transmission loss at the long wavelength side is apt to occur. In addition, with such an RDF, it suffers from the disadvantage that the polarization mode dispersion (PMD) also ends up generally becoming more than 0.07 ps/km1/2.
Further, the NZ-DSF type transmission line and SMF+RDF type transmission line using an SMF and RDF also suffer from the above disadvantages.
An object of the present invention is to provide a new type of a low loss dispersion-management transmission line overcoming the disadvantages of non-linearity and transmission loss.
According to the present invention, there is provided an optical transmission line having a positive dispersion optical fiber having a positive dispersion and a positive dispersion slope and a negative dispersion optical fiber having a negative dispersion and a negative dispersion slope and managing dispersion to have a non-zero dispersion (for example within xc2x15 ps/nm/km) in the 1.55 xcexcm band, wherein the transmission losses in the 1.55 xcexcm band of the positive dispersion optical fiber and negative dispersion optical fiber are both not more than 0.23 dB/km and the difference of the transmission losses of the positive dispersion optical fiber and negative dispersion optical fiber is not more than 0.05 dB/km.
The optical transmission line of the present invention (1) may be of a type where one span is comprised of a positive dispersion optical fiber and negative dispersion optical fiber, (2) may be of a type where one span is comprised of a positive dispersion optical fiber, negative dispersion optical fiber, and positive dispersion optical fiber so as to deal with two-way communications, or (3) may be of a type where one span is comprised of four or more fibers so as to keep down an increase in the cumulative dispersion. The configuration of the transmission line may be determined in accordance with the situation. In each case, more advantageous characteristics are obtained than the above loss characteristics.